This is a renewal application of a highly successful project that has previously elaborated postsynaptic regulation of opioid and dopamine receptors. The studies proposed in the renewal will examine also the presynaptic compartment, specifically focusing on Dl-type dopamine receptors that regulate the direct pathway GABAergic output from striatum / nucleus accumbens, and to precise evaluation ofthe regulation of synaptic vesicle exocytosis from axons. The Specific Aims of the proposed studies are: Specific Aim 1. Determine whether rapid DIR trafficking events are restricted to the somatodendritic surface or occur also in axons of MSNs. Specific Aim 2. Define the effects of extracellular DA dynamics and electrical activity on Dl R endocytosis in MSNs. Specific Aim 3. Identify funcfional consequences of DIR trafficking on pre- and post- synapfic signaling. The proposed experiments will define the basic properties of dopaminergic regulafion ofthe direct pathway, which controls natural motivated behavior and is critical for the reinforcing effects of addicfive drugs. The studies also directly test, and mechanistically elucidate, presynaptic regulafion by Dl receptors. This is a fundamental area of cellular neuroscience that is may reveal new approaches for manipulating addictove drug acfion or treating addicfive disorders. RELEVANCE (See instructions): Dopamine signaling in medium spiny neurons ofthe striatum control montivafion to natural sfimuli and mediate the reinforcing effects of addictive drugs. The proposed studies determin how this signaling works, and examines specific hypotheses for how it is regualted by natural stimuli relafive to drugs. This may idenfify specific features of addictive drug acfion for therapeutic targefing.